The present invention relates to polarizing agents for use in enhancing nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) signals via dynamic nuclear polarization (DNP) and, more particularly, to such agents that include two or more paramagnetic centers.
For about 50 years it has been known that it is possible to enhance the intensities of NMR signals by increasing the nuclear spin polarization with DNP. In this method, paramagnetic centers are introduced into a sample in a magnetic field and the large electron spin polarization of these centers is converted to nuclear spin polarization of nuclei within the sample via microwave irradiation. In this context, it is customary to utilize radicals such as the nitroxide 2,2,6,6,tetramethylpiperadine-1-oxyl (TEMPO), metal centers like Cr5+, or other monomeric paramagnetic centers. See, for example, Farrar et al., “Mechanism of Dynamic Nuclear Polarization in High Magnetic Fields,” Journal of Chemical Physics, 114(11): 4922-4933, 2001. It is also known that short-lived photo-excited triplet states can be used as polarizing agents in DNP experiments. See, Henstra, “High Dynamic Nuclear Polarization at Room Temperature,” Chemical Physics Letters, 165(1):6-10, 1990; and van den Heuvel, “Transient Oscillations in Pulsed Dynamic Nuclear Polarization,” Chemical Physics Letters, 188(34):194-200, 1992.